A diagram of a modern single lens reflex camera (SLR), illustrating how it forms an image on film.

• The lens elements focus the incoming light rays.
• Focused rays then pass through the diaphragm, a device that controls the amount of light passing through the lens.
• When the shutter release button is pressed, the mirror flips up and the shutter briefly opens to let the focused light rays strike the film.
• Focusing can be done manually or automatically. Focusing is necessary if you want a sharp image.
• The focused light reacts with a light-sensitive chemical coating on the film that records the real image. The image is upside down and smaller than the object but is larger once developed and printed.